1. Field of the Invention
The present invention relates generally to a valve drive device for an internal combustion engine, and more specifically to a valve drive device in which an oil pressure tappet, for automatically absorbing a gap created in a valve drive system, is built into a rocker arm, thereby preventing the wear of the contacting part of the oil pressure tappet exposed to heavy abrasion.
2. Description of the Prior Art
In a valve drive device in which a rocker arm is movably pivoted, one end thereof bearing the cam load and the other end thereof driving the valve stem, expansion and contraction of the valve stem due to the valve heat and wear of the valve seat cause a gap to be created between the valve drive transmitting parts. This gap, being liable to disturb the exhaust timing or to shorten the engine life, should be eliminated. It has been considered unwise to install an oil pressure tappet in the rocker arm, which is hydraulically moved to protrude or withdraw for the purpose of eliminating the gap, because it increases the moving weight and thereby decreases the maximum number of revolutions or results in the decrease of durability. If, however, the oil pressure tappet can be reduced in weight, it is desirable that the oil pressure tappet be built into the rocker arm since the whole valve drive device can then be simplified. This will also be desirable in that a gap-absorbing mechanism can be additionally obtained without any major design modification of the conventional overhead cam type drive mechanism.
For the purpose of building an oil pressure tappet into either the cam-contacting part or the valve stem-contacting part of the rocker arm and making the oil pressure tappet smaller, it is conceivable to insert a movable plate between the oil pressure tappet and the cam or the valve stem in contact therewith so that the frictional force acting on the oil pressure tappet from the cam or the valve stem in a direction normal to the central axis of the oil pressure tappet can be borne by the movable plate, thereby lessening the side thrust acting on the oil pressure tappet and shortening the required length of the oil pressure tappet. Smooth action and sufficient durability of a valve drive device with such an inserted movable plate depend on whether or not the slide-contacting part of the oil pressure tappet and the movable plate exposed to heavy abrasion with each other can be protected from seizure, wear and other damage.